1. Field of the Invention
The present invention concerns a mammography system with an x-ray source, a detector and a compression plate arranged in the beam path between these, as well as a method for operation of such a mammography system.
2. Description of the Prior Art
FIG. 7 shows an example of a conventional mammography system 2. The x-ray source 4, the detector 6 and the compression plate 8 are held on a vertical column 12 by a central shaft 10. The x-ray source 4 is a commercially available x-ray tube with a tungsten rotating anode. The detector 6 comprises a bearing plate (not shown in detail) for placement of the breast 14 to be examined. X-ray source 4, detector 6 and compression plate 8 together form the measurement system of the mammography system 2 and can be rotated around a common axis A relative to the vertical column 12. The measurement system is slid along the vertical column 12 to adapt the mammography system 2 to the size of the patient to be examined. In the following a female patient is referred to, however female and male patients are always meant.
For examination the breast 14 is initially compressed; this ensues via a displacement of the compression plate 8 in the beam direction of the x-ray source 4, and said breast 14 is subsequently irradiated by a beam of x-rays. The design of mammography system 2 shown in FIG. 7 establishes the distance between the x-ray source 4 and the detector 6 (which is also designated in the following as a tube-detector distance) known as: Source-Image Distance, SID 16. The tube-detector distance SID 16 is the distance between the location of the x-ray generation and the location of the detection of the x-rays. In a conventional x-ray apparatus, this is typically the distance between the surface of the anode of the x-ray tube from which the x-ray beam used for examination emanates and the x-ray-sensitive part of the detector, for example an x-ray film. This distance is also designated as a focus-detector distance.
Mammography exposures can be produced from various directions in which the patient respective adopts a different posture. Such mammography exposures are also designated as projections. The cranio-caudal projection (CC projection) or the mediolateral-oblique projection (MLO projection) are typical. FIGS. 8 and 9 show schematic frontal views of the mammography system 2. FIG. 8 shows an example of the acquisition geometry for an MLO projection. The entire measurement system is pivoted around a central axis A to change the projections.
In addition to conventional mammography, tomosynthesis as increasingly gained importance. In this examination method the breast 14 (held stationary in a compressed state) is irradiated from different directions (projection angles). To implement a tomosynthesis it is necessary that the compression of the breast 14 is decoupled from the movement of the x-ray tube 4. FIG. 10 shows an example of the movement progression of the x-ray tube 4 during the acquisition of a tomosynthesis image data set. During the acquisition the detector 6 and the compression plate 8 stand still while the x-ray tube 4 moves.
The mechanical design is very complicated, in particular of a mammography system 2 suitable for tomosynthesis. On the one hand, a mechanically stable acquisition of the measurement system is ensured, wherein this must likewise be height-adjustable for the adaptation to the size of the patient. To adjust the various projections (for example CC or MLO projection), the measurement system must additionally be attached to the vertical column 12 such that said measurement system can rotate. If the mammography system should moreover be suitable for tomosynthesis, an additional requirement is added, namely the decoupling of the movement of detector 6 and compression plate 8 from the movement of the x-ray source 4 as a mechanical requirement.